Hydrothermal reactions, i.e., reactions that take place in hot water at high pressures, have industrial and scientific importance in forming materials such as zeolites and large single crystals. A difficulty with hydrothermal reactions is the high rate of corrosion caused by the conditions necessary to conduct hydrothermal reactions. For example, corrosion of the material lining the walls of a hydrothermal reactor can cause contamination of the product and may even cause a catastrophic failure and explosion of the reactor vessel. Another difficulty is that catalyst support materials that are used in hydrothermal reactions can dissolve or change morphology in a sustained hydrothermal environment. Although ceramic materials have many desirable features for use in reactors, such as linings for reactor chambers and catalyst supports for aqueous phase reactions, ceramic materials are especially sensitive to the corrosive effects of hydrothermal conditions. See, for example, Wendlandt, et al., "The Reactions of Oxides with Water at High Pressures and Temperatures," Angew. Chem. Int'l. Ed., vol. 3, p47 (1964).
Great efforts have been devoted to the development of new refractory materials. For example, Brown in U.S. Pat. No. 4,057,433 describes a mold having a facing portion comprising finely divided particles of the oxyfluorides of the lanthanide and actinide series. Brown states that the finely divided particles have a particle size of from below 400 mesh to 5 mesh, i.e. having a particle size of from 0.1 to 4000 microns. In the examples section of U.S. Pat. No. 4,057,433 a particle size of 325 mesh (44 micron) is used. Preparations of rare earth oxyfluorides are known (see, e.g., Niihara et al, Bull. Chem. Soc. Jap., 44, 643 (1971) and deKozak, et al., Rev. de Chimie Miner., 17, 440 (1980); however, particles are obtained by pulverizing, and it is known that particle sizes of 1 micron and less are generally not obtainable by conventional grinding processes. Brown does not discuss grinding processes and does not discuss the morphology of the oxyfluorides nor their stability in hydrothermal conditions. In general, morphology and hydrothermal stability cannot be predicted based on a chemical formula since factors such as crystal growing conditions and crystal structure can have a strong influence on a material's properties. Moreover, there is no known correlation between a ceramic material's refractory properties and its performance in hydrothermal conditions.
Dugger in WO 93/17959 discloses that various oxyhalide complexes are useful precursors for making refractory oxides. Dugger does not disclose particle sizes of the oxyhalide precursors (other than passing through a screen size such as 200 mesh) and does not discuss the morphology of the oxyfluorides nor their stability in hydrothermal conditions.
In making ceramic articles it is frequently desirable to use small particles because, relative to larger particles, smaller particles are more reactive and sinter at lower temperatures. However, under hydrothermal conditions, small particles tend to dissolve and (in saturated solutions) deposit on a large crystal, such as a seed crystal, to form large crystalline materials. In other words, in a static cell, or equilibrium, hydrothermal conditions favor the dissolution of small particles and the formation of large particles.
Indeed, nanometer-sized particles have so far only been obtained from a supercritical aqueous environment by two types of non-equilibrium processes. The first approach has been to limit crystal growth by inducing an abrupt homogeneous nucleation of a dissolved solute in the region of an expanding jet (see, e.g., Smith, U.S. Pat. No. 4,734,451, Matson et al., J. Mat. Sci. 22, 1919 (1987) or the rapid thermal decomposition of precursors in solution. The second widely used non-equilibrium approach for production of nanoparticles employs a solid component as one of the reacting starting materials whereby the rate of dissolution limits the precipitation. Examples of the second approach are described in references such as Oota, et al., J. Cryst. Growth 46, 331 (1979) and Fedoseev, et al., Kristall und Technik 3, 95 (1968. Finally, at subcritical temperatures (200.degree. C.), whiskers of hydroxyapatite with widths of 0.1 to 1 micrometer (.mu.m) were obtained from a dilute solution of beta-Ca.sub.3 (PO.sub.4).sub.2 near the vapor pressure of the solution. See Yoshimura, et al., J. Mater. Sci., 29 3399 (1994).
There remains a need for hydrothermal processes for forming nanoparticles under equilibrium conditions and/or from a single phase solution. There is a further need for rare earth oxygen-containing fluoride nanoparticles. There is also a need for nanoparticles materials that are hydrothermally stable and/or have a fiber morphology.